Delete all posts by the sockpuppet when its account is merged with the suspended one. This should be a sign that what he does is absolutely useless. The posts shouldn't be there in the first place.
Restart the suspension counter at each breach. So if an account is suspended on 1 January for 12 months, and the user creates a sockpuppet to log in on 1 December of that year, the suspension period should be extended to 1 December of the next year.
You may not agree with 1), because the answer may be useful anyway. In that case there's no point in suspending, because you would prevent that user to post useful answers. Next, the user in question doesn't post good answers anyway (last 2 days: 5 upvotes, 15 downvotes). Next, if the sockpuppet wouldn't post an answer, someone else will. We have many competent users always willing to help.